(1) Field of the Invention
The present invention relates to towed acoustic line arrays and more particularly to a fiber optic bundle towed array having a relatively small diameter, while still providing individual steerable beams.
(2) Description of the Prior Art
Conventional towed arrays are generally built of ceramic piezoelectric transducers that are distributed and mounted within a tubular sheath. The construction, size (including diameter) and spacing of these ceramic transducer elements define the frequency band of operation of the array in a water medium. Conversely, the required operating frequency places restrictions on the minimum dimensions achievable using ceramic type array elements. The sheath type array described above is generally filled with any of several types of acoustically transmitting materials which provide structural integrity together with some measure of isolation from noise-producing turbulent flow. Since array diameter has a direct correlation with turbulent flow, it is desirable for the purpose of further reducing the noise effects of this turbulent flow to have the diameter of the array be as small as possible. Most present thin-line sheath type arrays have minimum diameters of approximately one inch except for co-pending U.S. patent application Ser. No. 547,273.
Furthermore in present towed arrays, in order to provide steerable beams, the amplitude and phase information from each array transducer element must be individually transmitted to beamformer electronics external to the array. This requires that along with the transducer elements themselves, at least one telemetry wire for each element must be packaged within the array sheath. In addition, other wires are required to deliver power to each piezoelectric ceramic array element. The need for such power supply and telemetry wires places further constraints on the minimum array diameter achievable and contributes to the very high production costs. Telemetry schemes that reduce the number of wires and hence wire bundle diameter require complicated circuitry to be contained in the array proper, thus again limiting the minimum cost and diameter achievable while potentially decreasing reliability. In summary, present towed arrays have larger than desired diameters and must contend with the concomitant high flow noise associated therewith, have high manufacturing costs and provide less than desired reliability.